On all tennis courts, perhaps the most notable part is the net. In one known example, a tennis net includes two main net posts placed at a pre-determined height (e.g., 107 cm), and positioned a distance (e.g., 91.4 cm) outside the outermost (or doubles) sidelines of a tennis court. The net is typically composed of a woven or plastic netting supported by an upper net cable. The netting hangs from a strong metal cable via a white net tape. The net cable is attached to one post and then to the opposite post, which has a crank that winds the cable so that the net tightens and rises up to the required height and a desired tension.
In the middle of the net, there is a center-strap (also known as a mid-strap) which holds the net down by coming over the top of the net and being fastened to a clasp on the playing surface. This provides greater tension than a crank could practically provide (by pulling down at the mid-point of the net), since cables generally will sag, and provides the defined low part of the net in the center, and at an determinable height, which is an important during play. The result is a semi-V shape running in the top part of the net, where the center strap provides the regulation 91.4 cm height of the net in the middle.
In many tennis tournaments throughout the world, each net can be different. Wimbledon is known for a loose net while the U.S. Open is known for having a tight highly tensioned net. In fact, because both tournaments are outdoors, the net may be taken down as much as several times on any particular day with inclement weather. From tournament to tournament, and court to court, and even from day to day, and hour to hour, there lacks a precise, uniform net tension in tennis. And with this varying net tension, comes varying net heights (as net height is directly correlated to net tension, as explained further below).
When net tension is different, balls that clip the very top of the net (“net tape”) during a rally will dribble over a loose net (resulting in what is called a cheap point), as a net with a looser tension absorbs the forward movement of the ball, allowing balls which hit the tape to roll over to the other side of the net. Conversely, with a tight net, the ball that clips the net tape will either sit up for an easy put away, or bounce out for a loss of the point, as tighter tensions do not absorb the forward pace and either send balls hitting the tape backward or cause the ball to change trajectory and fly out. This causes inconsistency in playing conditions.
Additionally, as mentioned, when net tension is different, the height of the net will be different. This is despite the fact that the net post is a defined height, as is the center-strap (107 cm and 91.4 cm respectively). The net traverses the court typically at 12.8 m in length (or 10.97 m for a singles net post). The net itself weighs approximately 10 kg (alternately approximately twice that for the ATP World Tour nets). Therefore, there will be a measurable sag in the net even when it is cranked up to reach the center-strap 91.4 cm height. The more one cranks thereafter, the tighter the net, thus as the net tension increases, the sag between the net posts/singles sticks and the center-strap will lessen, as the net cable will straighten out while being pulled tighter (resulting in different heights between those two points). This non-measured cranking (done essentially by “feel”) can result in not only an inconsistent net tension but also an inconsistent net height. While the height of a net is the same for players on both sides, a player with more top spin will have an advantage over a player who hits a flatter ball when the net height can vary as much as several millimeters over the most of the playing area of the net (and as much as 1 cm at the midpoint between the singles stick and center-strap where the sag is greatest). Further, players who prefer to hit down the line (as opposed to cross court) will have a lower/higher net height at precisely that part of the net, with potentially different results of any such shot, depending upon the net tension. As the court itself has strictly defined dimensions and measurements in millimeters, as determined by the tennis governing bodies, and thus define the height of the net at any given point by virtue of knowing (and creating) the net tension.
For serves, the current rules call for a “service let,” which is when the ball clips the net and still falls inside the service box. Loose nets will likely result in more lets while tight nets more likely cause the ball to either bounce back, sit up, or fly further and thus out (a “fault”). Professional tennis had recently considered removing the “service let.” Should this still happen in the future, more “aces” will occur with loose nets (as the ball dribbles over to the other side) while tight nets will cause more balls to sit up for easy winners by the opponent or will go fly out for a loss of the point. The result is different depending upon the net tension. This is unacceptable, as the effect can be different on different courts and even change on the same court on the same day when the net often needs to be taken down on outdoor courts during rain, or to change nets during events with different tours (which have different nets). The result is an ever varying net tension and height for virtually each time a net is set up. This is because the precise height of the net on either side of the centerstrap to each of the net posts (and thus the majority of the playing area) is dependent upon the net tension. Even after the tension is sufficient to pull the centerstrap up to its regulation 91.4 cm (3 ft) height, there remains a sag. Further cranking of the net not only increases the net tension but also straightens up the sag, which raises the net in varying degrees along its length, with the largest increase at the middle of the net between the centerstrap and net posts where most balls pass, and least amount moving closer to either the centerstrap or the net post. At this midpoint, the height differential between a loose net and a tight net can result in up to a centimeter or more discrepancy. Such a difference can have significant effects on play. Therefore, the need to regulate the net tension is more than just how the ball reacts after striking the top of the net cord. It determines the precise height of the net at the most critical parts of the court.